In a large saucepan, put a can of condensed milk and water. Boil for 3 hours. Making sure that the can is covered with water at all times. After 3 hours, remove the can from the hot water with kitchen tongs and allow to cool completely before using.
For the base: preheat the oven to 176°C/350°F. In a food processor, add flours, butter, egg and salt. Pulse 5 times. In a floured surface, knead the dough until it comes together. Grease 12 - 8cm, tart tins or a large one (24cm). For the individual pies: divide the dough into 12 portions; with the help of your fingertips, spread the dough covering the whole tin. Prick the bottom with a fork. For one big pie: On a floured surface, roll the dough, cover the pie tin with dough. Prick the base with a fork. Bake the individual pies for 15 minutes and for 20 minutes the big one. Let it cool completly.
Whipping cream: 10 minutes before beating, put in the freezer. Remove from the freezer, pour the cream in a medium bowl and add icing sugar. Beat with electric mixer until thick.
To assemble: divide the toffee among the 12 pie crusts. Add sliced bananas. Spread whipped cream evenly and if desired, grate a little chocolate on top.